Interferon α (IFNα: hereinafter, “interferon” is abbreviated as IFN) and interferon β (IFNβ) are known as type 1 IFNs which possess antiviral activity or antitumor activity. On the other hand, it has also been revealed that IFNα is related to autoimmune disease. For example, abnormal production of IFNα has been reported in patients with the following autoimmune diseases. It has also been suggested that symptoms of the autoimmune diseases can be reduced by neutralization of IFNα.    Systemic lupus erythematosus (Shiozawa et al., Arthr. & Rheum. 35, 412, 1992)    Chronic rheumatism (Hopkins et al., Clin. Exp. Immunol. 73, 88, 1988)
Cases in which symptoms of the autoimmune diseases had been manifested or worsened by administration of recombinant IFNα2 or IFN were reported (Wada et al., Am. J. Gastroenterol. 90, 136, 1995; Perez et al., Am. J. Hematol. 49, 365, 1995; Wilson L E et al, Semin Arthritis. Rheum. 32, 163-173, 2002).
Further, it has also been revealed that IFNα induces differentiation of dendritic cells. The dendritic cell is also an antigen presenting cell. Therefore, it is considered that the differentiation induction of dendritic cells consists an important mechanism in autoimmune diseases. It has been suggested that there is a deep association between the differentiation induction of dendritic cells of IFNα and the onset of systemic lupus erythematosus (Blanco et al., Science, 16:294, 1540-1543, 2001). Thus, it has been pointed out that IFNα is closely related to the antitumor activity as well as autoimmune diseases. In addition, IFNα is deeply involved in the onset of psoriasis (Nestle F O et al., J. Exp. Med. 202, 135-143, 2005).
Interferon Producing cells (IPCs) were identified as cells which produce type 1 IFN in large quantities associated with virus infection. Few IPCs are presented in the blood. It is considered that peripheral blood lymphocytes account for 1% or less of IPCs. However, IPCs have a very high capacity to produce IFN. IFN producing capacity of IPCs reaches, for example, 3000 pg/mL/104 cells. That is, it may be said that most of the IFNα or IFNβ in the blood, which is produced at viral infection, is resulted from IPCs, although there are few cells.
On the other hand, IPCs are undifferentiated lymphoid dendritic cells which are considered as precursor cells of dendritic cells. IPCs may be referred to as Plasmacytoid dendritic cells. IPCs are differentiated into dendritic cells by virus stimulation and induce the production of IFNγ or IL-10 by T cells. IPCs are also differentiated into dendritic cells by IL-3 stimulation. The differentiated dendritic cells by IL-3 stimulation induce the production of Th2 cytokine (IL-4, IL-5, and IL-10) by T cells. Thus, IPCs have properties which allow them to be differentiated into distinct dendritic cells by different stimulation.
Accordingly, IPCs have two profiles: IFN producing cells and precursor cells of dendritic cells. Both cells play an important role in immune system. In other words, IPC is one of the important cells which support immune system in various aspects.    Non-patent document 1: Shiozawa et al., Arthr. & Rheum. 35, 412, 1992    Non-patent document 2: Hopkins et al., Clin. Exp. Immunol. 73, 88, 1988    Non-patent document 3: Wada et al., Am. J. Gastroenterol. 90, 136, 1995    Non-patent document 4: Parez et al., Am. J. Hematol. 49, 365, 1995    Non-patent document 5: Bianco et al., Science, 16:294, 1540-1543, 2001    Non-patent document 6: Ju et al., Gene. 2004 Apr. 28; 331: 159-64.    Non-patent document 7: Colonna M et al., Seminars in Immunology 12: 121-127, 2000.    Non-patent document 8: Nakajima H. et al., J. Immunology 162: 5-8. 1999    Non-patent document 9: Wilson L E et al, Semin Arthritis. Rheum. 32, 163-173, 2002    Non-patent document 10: Nestle F O et al., J. Exp. Med. 202, 135-143, 2005    Patent-document 1: WO03/12061 (U.S. Patent Published Application No. 2003-148316)